Thermal Desorption Units and On-Column Injection Capillary and Packed Columns for Gas Chromatography Units have been successfully utilized in a variety of systems, methods and techniques for the separating, analyzing, identifying and recording both volatile and semi-volatile components in complex materials.
Capillary and packed columns for Gas Chromatography Units used in such systems, methods and techniques are generally classified by bore size. Thus, micro-bore capillary columns will have internal diameters, (I.D.'s), in the order of 0.25 mm or 0.32 mm; and macro-bore or mega-bore capillary columns will have an I.D. in the order of 0.53 mm or 0.75 mm. Packed Columns for Gas Chromatography Units have much larger I.D.'s in the order of 2.0 mm to 4.6 mm. The respective internal diameters (I.D.'s) will affect the quantities and rates at which carrier gas with the volatile or semi-volatile component to be identified and quantified is passed to the Gas Chromatography Unit.
Purge and trap thermal desorption gas chromatography-mass spectrometry is particularly suited to the isolation and identification of volatile components such as flavors in foods and beverages and other volatile and semi-volatile components in other complex matrices. The general method for use with this equipment requires at least the following steps:
1. Separation and Storage of the Component
Volatile components are sparged from numerous complex matrices such as gases, water, foods, beverages, cosmetics, petrochemicals and pharmaceuticals. These volatile components are passed into a sized collecting container having a predetermined trapping agent such as the porous polymer resin, 2, 6-diphenyl-p-phenyleneoxide sold under the trademark TENAX-TA or the activated graphitized carbon sold under the trademark CARBOTRAP for collecting and adsorbing thereon the particular volatile or semi-volatile component being determined. Both of the above enumerated trapping agents have a high affinity for non-polar organic compounds and a very low affinity for water vapor and other low molecular weight polar compounds such as alcohols with less than three carbon atoms.
While these trapping agents have been referred to and described, those skilled in the art will recognize that there are many other adsorbent materials which can function equally as well and that these trapping agents where referred to in the present Application are only by way of illustration and not intended to limit use of such other adsorbent materials as may also be used within the scope of the present invention.
With these trapping agents, when the sparged marked volatile or semi-volatile components are passed into the collecting container the water vapor will pass through the trapping agent and the volatile component will be adsorbed onto the surface of the trapping agent. Once the component is separated and trapped the sample is stored by sealing the sized collecting chamber until it is ready to be identified and quantified in any appropriate one of the enumerated gas chromatography-detector systems, methods or techniques.
2. Release and Delivery of the Component
The sized collecting container is placed in suitable apparatus where it can be heated to thermally desorb the volatile or semi-volatile components.
Then the desorbed volatile or semi-volatile component is passed or delivered by any suitable means to the gas chromatography unit where the volatile component and the marker compound are separated.
3. Monitoring Identification and Quantifying
By the operative coaction of the detector and its monitoring equipment with the gas chromatography unit, the separated volatile components are continuously monitored, and analyzed, and the results are recorded on and can be read out from the monitoring equipment. One such detector for this purpose is a mass spectrometer and reference will be made herein to such gas chromatography-mass spectrometer (GC-MS) by way of illustration only. However, those skilled in the art will recognize that the combination of the short path thermal desorption apparatus in accordance with the present invention with the gas chromatography unit is adapted for universal use in any of the gas chromatography-detector systems, methods and techniques, as was above enumerated, without departing from the scope and use of the present invention.
In older and currently known prior art desorption type purge and trap methodology a unitary device is used which includes, a purge and trap chamber, thermal desorption means for the marker compound and other volatile components of the complex matrix material being analyzed, transfer lines, and cryotraps. In these systems the adsorbent trap is an integral part of the apparatus and cannot be easily removed or replaced. It therefore is necessary to backflush the apparatus to clear it for the next sample to eliminate contamination and other side affects which will affect the nature and tend to degrade the component being analyzed.
In purge and trap methodology in which the desorption tube is desorbed in a heating chamber, the carrier gas will in addition to volatile component pickup artifacts which may be on the outside of the desorption tube, thus affecting the integrity of the sample component and the accuracy of the identification and quantification of the sample component.
These prior art systems do not work well for higher boiling point components or for general purpose volatile components, because they produce cross contamination between the samples being tested, have a memory effect between samples due to sample overload and repetitive analysis of many samples. Additionally, the transfer lines to the inlet port of the gas column of the gas chromatography unit are relatively long and this causes a measurable loss of resolution and produces some catalytic reaction which degrades labile component samples being analyzed.
The present invention overcomes these prior art problems by providing an improved short path thermal desorption apparatus for use in Gas Chromatography-Detector Techniques such as Gas Chromatography-Mass Spectrometry Systems and Techniques to provide a relative quick and accurate mechanism for achieving in each sample tested the desired qualitative and quantitative determination of either conventional components such as menthol in complex matrices such as food, or more complex components such as fatty acids, high boiling point lipid peroxidation species, less volatile pyrazines, terpenes, sesamol from sesame, thiols, dithiols, disulfides, trisulfides, thoiesters, pheromones, pesticides, and other high boiling point impurities in such complex materials or matrices.
Such short path thermal desorption apparatus in accordance with the present invention provides at least several primary advantages. First, it enables the sample component in a glass lined or fused silica lined stainless steel tube type adsorbent trap to be subjected to ballistic type heating. Second, the desorbed component can be transferred easily and efficiently into the injection port for the gas column of the gas chromatography unit through transfer lines consisting of the short stainless steel type adsorbent trap and its associated needle injection assembly wherein the walls are either coated with a boro-silicate or are fused silica lined, thereby providing in the short transfer path for the sample component an inert environment which minimizes degradation of labile sample components which often decompose upon contact with hot catalytic metal wall surfaces of the transfer path. And third, each sample component has its own individual and new stainless steel tube adsorbent trap and associated needle injection assembly, having an inlet and outlet for the independent charging of fresh carrier gas therethrough to eliminate the possibility of cross-contamination from sample component to sample component, thus preventing any "memory effect" which has heretofore occurred in the prior art desorbing apparatus due to overloading of the adsorbent in the stainless steel type adsorbent trap, as will now be more fully described.
Ballistic Heating when used herein means heating with a sharp rise in the temperature of the desorption tube, at an approximate rate or ramp speed of about 100.degree. C. per minute, to enable the desorption tube to be brought rapidly to the boiling temperature of the volatile or semi-volatile component to be desorbed from the desorption tube.